Story highlights

The Kiev mayor is among four under investigation for excessive use of violence

Police estimate 57,000 people attend pro-government rally

U.S. Sen. John McCain took in the anti-government protest scene in Kiev's Independence Square, known as the Maidan, on Saturday, snapping pictures with his cell phone as he met with opposition leaders.

He called it an "incredible display of patriotism" in a tweet that includes an aerial photo of the square. McCain also said he was with the younger brother of heavyweight boxing champion Vitali Klitschko, who is one of the opposition leaders.

The Republican senator has been critical of Ukrainian authorities' use of force against protesters.

"If Ukraine's government thinks that brute force and the politics of fear can see it through the current crisis, it is woefully mistaken," McCain said in a statement last week.

Photos: Ukraine protests 77 photos

Photos: Ukraine protests77 photos

Ukraine protests – Newlyweds Mikhail and Margarita Nakonechniy kiss in front of barricades on Independence Square in a gesture of support for pro-Europe activists in Kiev, Ukraine, on Saturday, December 21. Protesters have poured into the streets of the Ukrainian capital, angered by their government's move away from the European Union in favor of closer ties with Russia.

Ukraine protests – A girl wearing a traditional Ukrainian wreath attends an opposition rally in Independence Square on December 17.

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Ukraine protests – A young girl stands next to police officers guarding the presidential offices in central Kiev on December 17.

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Ukraine protests – Protesters sit behind wood bricks in Independence Square on December 17. Written on the bricks are names of Ukrainian settlements whose inhabitants are taking part in the demonstrations.

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Ukraine protests – Flowers are stuck on barriers in front of Ukrainian Interior forces guarding Kiev's administrative district on December 17.

Ukraine protests – Activists warm themselves beside a bonfire as they guard barricades at Independence Square on Monday, December 16.

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Ukraine protests – Pro-EU activists cook during a rally in Independence Square on Sunday, December 15.

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Ukraine protests – U.S. Sens. John McCain, center, and Chris Murphy, right, join Ukrainian opposition leader Vitali Klitschko during a mass rally at Independence Square on December 15. McCain told protesters seeking closer ties with Europe, "The free world is with you, America is with you."

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Ukraine protests – An estimated 200,000 pro-EU activists gather during a rally in Independence Square on December 15.

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Ukraine protests – People sing, shout slogans and wave Ukrainian and EU flags during a mass rally in Independence Square on December 15.

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Ukraine protests – Pro-EU activists wave flags around the monument to the founders of Kiev during a rally in Independence Square on December 15.

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Ukraine protests – A couple of pro-EU activists share a tender moment at a tent camp in Kiev on December 15.

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Ukraine protests – Pro-EU activists, one waving the EU flag, gather during a rally in Kiev on December 15.

Ukraine protests – A protester shouts slogans and waves a flag of the Batkivshchyna party during a protest near the Ukrainian Security Service building in Kiev on December 15.

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Ukraine protests – Ukrainian opposition supporters gather at a mass rally in Kiev on December 15.

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Ukraine protests – Demonstrators by the thousands rally in Independence Square on Saturday, December 14.

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Ukraine protests – People go through a police line separating the competing rallies of opposition protesters in Independence Square and supporters of Ukrainian President Viktor Yanukovych in Kiev on December 14.

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Ukraine protests – Supporters of the president wave flags of the ruling Party of Regions, as well as Ukrainian flags, during a rally on Kiev's European Square on December 14.

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Ukraine protests – Anti-government protesters guard a barricade designed to keep police from evicting them from Independence Square on Friday, December 13.

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Ukraine protests – Protesters stand at a barricade in Kiev on December 13.

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Ukraine protests – Protesters hold Ukrainian national flags as they picket the Ukrainian House during round-table talks between Yanukovych, former presidents and leaders of the opposition on December 13.

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Ukraine protests – Riot police storm barricades set up by pro-EU protesters in Independence Square on Wednesday, December 11.

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Ukraine protests – Riot police move against protesters on Independence Square around 2 a.m. December 11. U.S. Secretary of State John Kerry expressed "disgust" over the crackdown.

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Ukraine protests – A protester calls for attention inside Kiev's City Hall after riot police were forced out from blocking the front door on December 11.

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Ukraine protests – Police enter Independence Square on December 11.

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Ukraine protests – Riot police officers block the entrance of Kiev's City Hall as protesters pour water from windows and throw smoke grenades at them on December 11.

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Ukraine protests – A young police officer guards a roadblock on Institutskaya Street near the presidential area in Kiev on December 11.

Ukraine protests – A demonstrator holds a torn portrait of Yanukovych on November 29.

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Ukraine protests – Demonstrators hold hands during a rally on November 29.

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Ukraine protests – Demonstrators take to the streets in the center of Kiev on November 28.

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EXPAND GALLERY

Ukrainian prosecutors are investigating some top government officials regarding the beating of students at anti-government protests two weeks ago.

The subjects of the criminal investigation include Ukrainian Deputy Secretary for National Security Vladimir Sivkovich, Kiev Police Chief Valery Koryak and Kiev Mayor Oleksandr Popov. A fourth person also is under investigation.

The case will be put before a judge on Monday, who could order the those being investigated to house arrest, said Valery Vilkova, spokeswoman for the general prosecutor.

The tumult in Ukraine goes to the heart of its future ties with Russia and the rest of Europe.

Protests have unfolded in Ukraine since November 21 after President Viktor Yanukovych changed his stance on a trade pact with the European Union that had been years in the making.

He refused to sign the agreement, which would have strengthened cooperation with the European Union. Instead, he opted for closer ties with neighboring Russia.

Hundreds of thousands of Ukrainians have taken to the streets of Kiev to demand that the EU deal be signed.

The demonstrations are the biggest the country has seen since the Orange Revolution overturned the results of the country's 2004 presidential election. Protesters have created camps and barricades throughout the city center, and have set up makeshift soup kitchens, said CNN iReporter Harvey Carrol.

The epicenter of the protest has been in the city's central Maidan Square, where protesters have created a giant "revolutionary" Christmas tree decorated with flags and a giant poster of jailed former Prime Minister Yulia Tymoshenko.

On Saturday, a pro-government rally was held at nearby European Square, according to the state news agency Ukrinform. Kiev police estimated about 57,000 people participated in the rally. Ukrainian Prime Minister Mykola Azarov spoke at the gathering.